Allocation of Carbon from an Arbuscular Mycorrhizal Fungus, Gigaspora margarita , to Its Gram-Negative and Positive Endobacteria Revealed by High-Resolution Secondary Ion Mass Spectrometry.

Arbuscular mycorrhizal fungi are obligate symbionts of land plants; furthermore, some of the species harbor endobacteria. Although the molecular approach increased our knowledge of the diversity and origin of the endosymbiosis and its metabolic possibilities, experiments to address the functions of the fungal host have been limited. In this study, a C flow of the fungus to the bacteria was investigated. Onion seedlings colonized with Gigaspora margarita , possessing Candidatus Glomeribacter gigasporarum ( Ca Gg, Gram-negative, resides in vacuole) and Candidatus Moeniiplasma glomeromycotorum ( Ca Mg, Gram-positive, resides in the cytoplasm,) were labelled with 13 CO 2 . The 13 C localization within the mycorrhiza was analyzed using high-resolution secondary ion mass spectrometry (SIMS). Correlative TEM-SIMS analysis of the fungal cells revealed that the 13 C/ 12 C ratio of Ca Gg was the lowest among Ca Mg and mitochondria and was the highest in the cytoplasm. By contrast, the plant cells, mitochondria, plastids, and fungal cytoplasm, which are contributors to the host, showed significantly higher 13 C enrichment than the host cytoplasm. The C allocation patterns implied that Ca Mg has a greater impact than Ca Gg on G. margarita , but both seemed to be less burdensome to the host fungus in terms of C cost.